Syphilis, a classical sexually transmitted disease (STD), remains a significant public health problem in the United States and globally. Cyclic and regional epidemiologic trends, coupled with the fact that syphilitic genital ulcers are cofactors for the transmission of HIV, underscore the importance of continued studies to elucidate the complex pathogenesis epidemiology, and immunology of syphilis. Though one of the oldest recognized STDs, syphilis remains among the most poorly understood, largely due to the fact that Treponema pallidum cannot be cultivated in vitro. Regarding contemporary syphilis research, it is believed that the identification of T. pallidum outer membrane proteins (OMPs) potentially represents the "Holy Grail" of syphilis immunology;that is, once identified, they may be formulated into vaccines that could elicit bactericidal antibodies. However, the identification of T. pallidum OMPs has been highly problematic;to date, not a single OMP of T. pallidum has been definitively identified. Nonetheless, the indentification of OMPs of T. pallidum remains a laudable goal of syphilis research that must continue. To this end, Specific Aim 1 of this proposal will implement a new, highly sensitive, surface-labeling technique to identify candidate OMPs in T. pallidum. Three candidates already have been identified in preliminary studies. Putative OMPs identified in this manner, along with other candidates, will be subjected to more rigorous membrane topology studies in T. pallidum (Specific Aim 2). Finally, past attempts to demonstrate the efficacy of selected recombinant treponemal antigens in classical vaccine studies with rabbits have not been fruitful. To move syphilis vaccine development to the next level of more modern vaccinology, we will apply contemporary genetic immunization strategies to rabbits (Specific Aim 3) using OMP candidates emanating from Specific Aims 1 and 2. We already have evidence that genetically engineered treponemal proteins can be expressed in rabbit cells. The combined studies will substantially advance our knowledge of the relationships between T. pallidum membrane architecture, syphilis pathogenesis, and protective immunity.